The present invention relates to the making of stepped hollows, each having portions of significantly different cross sectional profiles and contours.
Hollows of the type to which the invention pertains are used, for example, for single-piece or profiled driving shafts. A hollow of such a variety is, for example, made by deforming a tubular blank under internal pressure in a closable, profiled die. The deforming commences freely, i.e., without support from the outside of the hollow, until abutting the wall of the die. If internal pressure is the only deforming force that is being applied, great differences in the final cross section cannot be obtained in that manner. In the case of relatively thin-walled hollows, the ratio of widening, (D-d/d), is limited by the stress in the apex point of tear strength in a stress-strain diagram as applicable to the particular material. Also, this known method produces a final wall thickness that is locally determined by the diameter ratio.
Other methods are known in which internal pressure is combined with an axial compression force acting on the tubular blank. Wall thickness raduction can be controlled to some extent, obviating the last-mentioned problem; but, by and large, the approach is also not completed satisfactorily. Nevertheless, the ratio between large and small diameter (bulging) can be 1.2- to 2-fold the value of the extension or elongation. However, tearing of the tube or formation of folds have to be avoided; this requires that the length of the bulge be limited to twice the diameter of the tube (l&lt;2d).